1. Field of the Invention
The present invention generally relates to a power supply device, and a light-emitting diode (LED) device and electronic device using the power supply device.
2. Discussion of the Background Art
To supply power to a load, such as a LED, a power supply circuit that includes a constant-current circuit connected to a driving path for the load, and a step-up circuit for driving the load is currently used. To enhance efficiency in power supply by such a power supply circuit, changes in a drive state of the load are monitored and the voltage step-up rate of the step-up circuit is controlled based on results of the monitoring.
In a known method, when voltage from a power source decreases, driving voltage is maintained constant by using the step-up circuit to increase the driving voltage for the load (LED) so as to enhance efficiency in power supply and/or reduce power consumption.
However, the power source voltage may be increased by supplying power, such as by charging, or electrical current of the load may decrease, and accordingly a forward voltage of the load may decrease while the load is driven by the driving voltage increased by the step-up circuit. If the increased driving voltage is continuously applied to the load in this state, the load receives an excessive voltage and efficiency in power supply is reduced.
Therefore, a need has arisen for optimizing the driving voltage for the load by controlling the voltage step-up rate of the step-up circuit so as to correspond to changes in the power source voltage and the drive state of the load.